The Philippines expressed “serious concerns” over the presence of China’s strategic bombers in the disputed South China Sea and its foreign ministry has taken “appropriate diplomatic action”, the spokesman of President Rodrigo Duterte said on Monday.

China’s air force said bombers such as the H-6K had landed and taken off from islands and reefs in the South China Sea as part of training exercises last week, drawing angry reactions from opposition lawmakers in Manila. The United States also sent ships to the disputed areas.

However, the foreign ministry stopped short of condemning China’s action, which Washington said could raise tension and destabilize the region.

In Beijing, a foreign ministry spokesman urged other countries not to over-interpret what he called a routine military patrol.